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Study of the effect of the medium on the entropic and enthalpic reaction constants for the dissociation of nitro-phenols in water—dimethylsulfoxide mixtures at 25°C
Thermochimica Actu, 64 (1983) 187- 194 Elsevier Science Publishers B.V., Amsterdam
187 - Printed
in The Netherlands
STUDY OF THE EFFECT OF THE MEDIUM ON THE ENTROPIC AND ENTHALPIC REACTION CONSTANTS FOR THE DISSOCIATION OF NITRO-PHENOLS IN WATER-DIMETHYLSULFOXIDE MIXTURES AT 25°C
F. RODANTE,
M.G. BONICELLI
and G. CECCARONI
Istituto dr Chimica, Facoltd! dr Ingegneria: Unrversitci dt Roma, Rome (Itab) (Received
4 November
1982)
ABSTRACT The effect of the medium on the reaction constants of the dissociation of nitro-phenols in water-dimethylsulfoxide mixtures ranging from 0.0 to 0.8 mole fraction of DMSO was further investigated. Some more correct reaction constants together with the entropic (ps) and enthalpic ( nH) reaction constants give a more complete picture of the effect of the medium.
INTRODUCTION
The effect of the medium on the reaction constants of the dissociation of nitro-phenols in water-dimethylsulfoxide (DMSO) mixtures has been examined previously [ 11. The ionization processes of nitro-benzoic acids at the same mole fractions were used as standard reactions. In this research some linear free energy relationships were used (NO,PhOH)
X + (PhO-)
(NO,PhCOOH)
X = (NO,PhO-)
X + (PhCOO-)
X + (PhOH) X
X = (NO,PhCOO-)
X + (PhCOOH)
- SAG0 = 2.303RTpa -aAGo= -SAG0
2.303RTp(a
(1) X
(2) (3)
+ RAD~-)
Aep = (IP
-
(4)
UP
= pOu,,+ fF + SE, + rR
(5)
where X (ranging from 0.0 to 0.8) represents the mole fraction of the solvent. Some different equations were used for the meta, para and ortho derivatives. Indeed the nitro group in the para position exerts a much greater influence on the substituent constant of phenol than it does on that of benzoic acid because the reaction center involves an atom with unshared electron pairs and it is directly conjugated with the strong withdrawing group NO,. 0040-603 l/83/$03.00
Q 1983 Elsevier Science Publishers
B.V.
188
Furthermore, ortho-nitro-phenol has been shown to take part in a strong internal hydrogen bond. It could be concluded that such equations represent the reaction constants of the three isomers. quite well, but in order to calculate the pK,,, values of phenol at 25OC the AH,” values of m-nitro-pheno1 were used in the equation PK,%
A H&AT = PK,ooc - 1 987 x 2 3(,3T T
12
because no AH:, values for the ionization of phenol in the various water-DMSO mixtures at 25°C were found [l]. For this reason neither enthalpic nor entropic contributions of the substituent effect on the reaction constants have been calculated. Since the AH0 and AS0 values of the ionization of phenol have recently been obtained in this laboratory [2], a more complete picture of the ionization of the isomers of nitro-phenol can be supplied. This is possible by separating the substituent effects into enthalpic and entropic contributions -
i3AG” 2.303RT = PHUH+PS%
pHaH and psus being defined
(6) by the relations
6AH” pHaH = - 2.303RT
(64
6AS0 psus = 2.303R
(6b)
It is clear that some different relations must be used to account for the different behavior of the nitro group in the three isomers. For the mera isomers eqns. (6a) and (6b) are satisfactory, while the para isomers need the relations -
AH0 = PHUH+ pRAaPw 2.303RT
(64
and
(6b) because the resonance effect influences isomers require the relations -
the enthalpic
term. Finally,
the ortho
AH0 = rR + SE, + p@H 2.303RT
(64
AS0 ---=p,a,+fF 2.303R because the inductive
(64 effects are entropy
controlled,
and the secondary
steric
189
effect influences term [l].
EXPERIMENTAL
the resonance
effect
which, in turn, affects
the enthalpic
AND PROCEDURE
The calorimetric apparatus, preparation of the DMSO-water solutions and the technique for obtaining the thermodynamic values for nitro-phenols and phenol have been described previously [2,3].
RESULTS
AND DISCUSSION
Using the experimental ionization values of phenol and nitro-phenols, standard free energy change for eqns. (3), (4) and (5) is obtained as
the
SAG’ = AGO,oZphoW- AGO,,,, In the same way, 6AHo and SAS’ can be calculated. The a,,,, up and a, values for nitro-benzoic acids have been calculated previously in the various mole fractions [4,5]. The uu and us values for the three isomers are also available [4,5]. Thef,F,R,r, S and Es values in the same mole fractions have been reported previously [ 1,4,5] These values and the new SAG’ values for the three nitro isomers are included in eqns. (3), (4) and (5). Thus, some more correct values for the reaction constants of the above-mentioned compounds, in the whole mole fraction range, are obtained and reported in Table 1.
TABLE
1
p Values calculated mixtures at 25°C x
DMSO
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
using eqns.
(3)-(5)
for m-, p-, and o-nitro-phenols
P
m-Nitrophenol
p-Nitrophenol
2.17 3.01 3.11 3.67 3.74 4.47 4.77 4.67 4.49
2.28 2.54 2.80 3.19 3.59 4.16 4.42 4.51 4.41
o-Nitrophenol 1.91 2.40 ’ 2.70 3.07 3.53 4.31 5.12 5.08 5.30
in water-DMSO
Fig. 1. Variation of the p and Ap values for m- (0). p- (0) function of the mole fraction of DMSO at 25°C.
and o-nitro-phenols
(a) as a
The values of the reaction constants, p, and the variations, Ap, with respect to their previous values [l] are plotted in Fig. 1. The trend of the three reaction constants remains similar to that previously found [l] for x nMSo G 0.6. Beyond this mole fraction, the Ap values start to_modify the shapes of the curves. Equations (6a), (6b), (6c), (6d) and (6e) give the pHa, and psas values for m-, p- and o-nitro-phenol, respectively. From these values, it is also possible to calculate the enthalpic (pu) and entropic ( ps) reaction constants for the three isomers (Tables 2-4). The values of the enthalpic reaction constants, pu (see Fig. 2), are larger (mostly for the ortho and paru derivatives) than those of the entropic reaction constants, ps (see Fig. 3). This indicates that for the reaction
191 TABLE 2 p Values calculated using enthalpic [eqn. (6d)] o-nitro-phenol in water + DMSO mixtures at 25°C
and
entropic
[eqn.
(6e)] equations
for
p Values calculated using enthalpic [eqn. (6a)] and m-nitro-phenol in water-DMSO mixtures at 25°C
entropic
[eqn.
(6b)]
equations
for
entropic
[eqn.
(6b)]
equations
for
X DMSO
PH
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
TABLE
-
Ps
-6.18 10.10 19.14 16.69 24.65 58.43 - 5.37 8.86 5.08
-
0.61 0.46 2.26 1.10 2.64 6.35 3.08 6.21 5.56
3
X DMSO 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
Ps
PH
- 2.09 - 7.08 - 22.36 2.54 - 3.34 - 5.69 11.73 20.51 35.51
1.33 0.86 -2.61 3.65 2.34 2.41 6.76 8.71 11.93
TABLE 4
p Values calculated using enthalpic [eqn. (6c)] and p-nitro-phenol in water-DMSO mixtures at 25’C X DMSO
PH
Ps
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
2.36 - 2.43 - 7.20 - 0.30 - 29.02 -2.17 10.97 53.36 27.17
2.44 1.53 0.66 2.21 8.41 4.36 6.10 13.35 8.80
192
Fig. 2. Variation of the pH values for the o- (a), m- (Cl) and p-nitro-phenols (0) of the mole fraction of DMSO at 25T.
as a function
constants the enthalpic contribution prevails over the entropic contribution, as could be expected by virtue of the greater delocalization shown by the nitro group in the phenate ion [ 11. Furthermore, it is noteworthy that the ps values for the ortho derivatives are negative in the mole fraction range 0.0-0.6 and show a maximum at X nMsO = 0.5. This pattern can be explained bearing in mind that the solute-solvent interactions are expressed by means of the entropic reaction constants, ps. Thus the greater desolvation of the phenol anions with respect to the benzoic anions influences the shape of ps curves. The differences [2,6] in the solvation enthalpy among the nitro-benzoic and nitro-phenol anions,
193
Fig. 3. Variation of the ps values for the o- (A),m- (0) and p-nitro-phenols of the mole fraction of DMSO at 25T.
(0)
as a function
-12 _
Z-16
-
<
m
ho -
JH
(NO,PhCOO-I-iNO,PhO-1
Fig. 4. Differences in the solvation enthalpies between nitro-benzoic and nitro-phenol in various DMSO-water mixtures at 25°C. (A) o-, (0) m-, and (0) p- nitro-phenol.
anions
194
are plotted in Fig. 4. As previously shown [ 11,the p values measure the sensitivity of the phenol derivatives to the electron withdrawing effect of the nitro group with respect to the benzoic derivatives in the same solvent. It is also known [l] that the solvent interferes to a minor extent with the interactions between the reaction site and the substituent group of the nitro-phenols. The pn and ps curves give a more complete picture of the effect of the medium on the reaction constants. At X,,,, = 0.5, where the charge delocalization of the ortho phenol anion is maximum, its solvation is maximum [l]. qkO,PhCOO,,
-(N02PhO_)x,
ACKNOWLEDGEMENT
This work was carried out with the financial support of the Cons&ho Nazionale delle Ricerche (C.N.R.) of Italy.
REFERENCES 1 2 3 4 5 6
F. F. F. F. F. F.
Rodante, Rodante, Rodante, Rodante, Rodante, Rodante,
Thermochim. G. Ceccaroni Thermochim. Thermochim. Thermochim. Thermochim.
Acta, 38 (1980) 311. and M.G. Bonicelli, Thermochim. Acta, 64 (1983) 179. Acta, 23 (1978) 311. Acta, 34 (1979) 29. Acta, 34 (1979) 377. Acta, 32 (1979) 293.
187 - Printed
in The Netherlands
STUDY OF THE EFFECT OF THE MEDIUM ON THE ENTROPIC AND ENTHALPIC REACTION CONSTANTS FOR THE DISSOCIATION OF NITRO-PHENOLS IN WATER-DIMETHYLSULFOXIDE MIXTURES AT 25°C
F. RODANTE,
M.G. BONICELLI
and G. CECCARONI
Istituto dr Chimica, Facoltd! dr Ingegneria: Unrversitci dt Roma, Rome (Itab) (Received
4 November
1982)
ABSTRACT The effect of the medium on the reaction constants of the dissociation of nitro-phenols in water-dimethylsulfoxide mixtures ranging from 0.0 to 0.8 mole fraction of DMSO was further investigated. Some more correct reaction constants together with the entropic (ps) and enthalpic ( nH) reaction constants give a more complete picture of the effect of the medium.
INTRODUCTION
The effect of the medium on the reaction constants of the dissociation of nitro-phenols in water-dimethylsulfoxide (DMSO) mixtures has been examined previously [ 11. The ionization processes of nitro-benzoic acids at the same mole fractions were used as standard reactions. In this research some linear free energy relationships were used (NO,PhOH)
X + (PhO-)
(NO,PhCOOH)
X = (NO,PhO-)
X + (PhCOO-)
X + (PhOH) X
X = (NO,PhCOO-)
X + (PhCOOH)
- SAG0 = 2.303RTpa -aAGo= -SAG0
2.303RTp(a
(1) X
(2) (3)
+ RAD~-)
Aep = (IP
-
(4)
UP
= pOu,,+ fF + SE, + rR
(5)
where X (ranging from 0.0 to 0.8) represents the mole fraction of the solvent. Some different equations were used for the meta, para and ortho derivatives. Indeed the nitro group in the para position exerts a much greater influence on the substituent constant of phenol than it does on that of benzoic acid because the reaction center involves an atom with unshared electron pairs and it is directly conjugated with the strong withdrawing group NO,. 0040-603 l/83/$03.00
Q 1983 Elsevier Science Publishers
B.V.
188
Furthermore, ortho-nitro-phenol has been shown to take part in a strong internal hydrogen bond. It could be concluded that such equations represent the reaction constants of the three isomers. quite well, but in order to calculate the pK,,, values of phenol at 25OC the AH,” values of m-nitro-pheno1 were used in the equation PK,%
A H&AT = PK,ooc - 1 987 x 2 3(,3T T
12
because no AH:, values for the ionization of phenol in the various water-DMSO mixtures at 25°C were found [l]. For this reason neither enthalpic nor entropic contributions of the substituent effect on the reaction constants have been calculated. Since the AH0 and AS0 values of the ionization of phenol have recently been obtained in this laboratory [2], a more complete picture of the ionization of the isomers of nitro-phenol can be supplied. This is possible by separating the substituent effects into enthalpic and entropic contributions -
i3AG” 2.303RT = PHUH+PS%
pHaH and psus being defined
(6) by the relations
6AH” pHaH = - 2.303RT
(64
6AS0 psus = 2.303R
(6b)
It is clear that some different relations must be used to account for the different behavior of the nitro group in the three isomers. For the mera isomers eqns. (6a) and (6b) are satisfactory, while the para isomers need the relations -
AH0 = PHUH+ pRAaPw 2.303RT
(64
and
(6b) because the resonance effect influences isomers require the relations -
the enthalpic
term. Finally,
the ortho
AH0 = rR + SE, + p@H 2.303RT
(64
AS0 ---=p,a,+fF 2.303R because the inductive
(64 effects are entropy
controlled,
and the secondary
steric
189
effect influences term [l].
EXPERIMENTAL
the resonance
effect
which, in turn, affects
the enthalpic
AND PROCEDURE
The calorimetric apparatus, preparation of the DMSO-water solutions and the technique for obtaining the thermodynamic values for nitro-phenols and phenol have been described previously [2,3].
RESULTS
AND DISCUSSION
Using the experimental ionization values of phenol and nitro-phenols, standard free energy change for eqns. (3), (4) and (5) is obtained as
the
SAG’ = AGO,oZphoW- AGO,,,, In the same way, 6AHo and SAS’ can be calculated. The a,,,, up and a, values for nitro-benzoic acids have been calculated previously in the various mole fractions [4,5]. The uu and us values for the three isomers are also available [4,5]. Thef,F,R,r, S and Es values in the same mole fractions have been reported previously [ 1,4,5] These values and the new SAG’ values for the three nitro isomers are included in eqns. (3), (4) and (5). Thus, some more correct values for the reaction constants of the above-mentioned compounds, in the whole mole fraction range, are obtained and reported in Table 1.
TABLE
1
p Values calculated mixtures at 25°C x
DMSO
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
using eqns.
(3)-(5)
for m-, p-, and o-nitro-phenols
P
m-Nitrophenol
p-Nitrophenol
2.17 3.01 3.11 3.67 3.74 4.47 4.77 4.67 4.49
2.28 2.54 2.80 3.19 3.59 4.16 4.42 4.51 4.41
o-Nitrophenol 1.91 2.40 ’ 2.70 3.07 3.53 4.31 5.12 5.08 5.30
in water-DMSO
Fig. 1. Variation of the p and Ap values for m- (0). p- (0) function of the mole fraction of DMSO at 25°C.
and o-nitro-phenols
(a) as a
The values of the reaction constants, p, and the variations, Ap, with respect to their previous values [l] are plotted in Fig. 1. The trend of the three reaction constants remains similar to that previously found [l] for x nMSo G 0.6. Beyond this mole fraction, the Ap values start to_modify the shapes of the curves. Equations (6a), (6b), (6c), (6d) and (6e) give the pHa, and psas values for m-, p- and o-nitro-phenol, respectively. From these values, it is also possible to calculate the enthalpic (pu) and entropic ( ps) reaction constants for the three isomers (Tables 2-4). The values of the enthalpic reaction constants, pu (see Fig. 2), are larger (mostly for the ortho and paru derivatives) than those of the entropic reaction constants, ps (see Fig. 3). This indicates that for the reaction
191 TABLE 2 p Values calculated using enthalpic [eqn. (6d)] o-nitro-phenol in water + DMSO mixtures at 25°C
and
entropic
[eqn.
(6e)] equations
for
p Values calculated using enthalpic [eqn. (6a)] and m-nitro-phenol in water-DMSO mixtures at 25°C
entropic
[eqn.
(6b)]
equations
for
entropic
[eqn.
(6b)]
equations
for
X DMSO
PH
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
TABLE
-
Ps
-6.18 10.10 19.14 16.69 24.65 58.43 - 5.37 8.86 5.08
-
0.61 0.46 2.26 1.10 2.64 6.35 3.08 6.21 5.56
3
X DMSO 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
Ps
PH
- 2.09 - 7.08 - 22.36 2.54 - 3.34 - 5.69 11.73 20.51 35.51
1.33 0.86 -2.61 3.65 2.34 2.41 6.76 8.71 11.93
TABLE 4
p Values calculated using enthalpic [eqn. (6c)] and p-nitro-phenol in water-DMSO mixtures at 25’C X DMSO
PH
Ps
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
2.36 - 2.43 - 7.20 - 0.30 - 29.02 -2.17 10.97 53.36 27.17
2.44 1.53 0.66 2.21 8.41 4.36 6.10 13.35 8.80
192
Fig. 2. Variation of the pH values for the o- (a), m- (Cl) and p-nitro-phenols (0) of the mole fraction of DMSO at 25T.
as a function
constants the enthalpic contribution prevails over the entropic contribution, as could be expected by virtue of the greater delocalization shown by the nitro group in the phenate ion [ 11. Furthermore, it is noteworthy that the ps values for the ortho derivatives are negative in the mole fraction range 0.0-0.6 and show a maximum at X nMsO = 0.5. This pattern can be explained bearing in mind that the solute-solvent interactions are expressed by means of the entropic reaction constants, ps. Thus the greater desolvation of the phenol anions with respect to the benzoic anions influences the shape of ps curves. The differences [2,6] in the solvation enthalpy among the nitro-benzoic and nitro-phenol anions,
193
Fig. 3. Variation of the ps values for the o- (A),m- (0) and p-nitro-phenols of the mole fraction of DMSO at 25T.
(0)
as a function
-12 _
Z-16
-
<
m
ho -
JH
(NO,PhCOO-I-iNO,PhO-1
Fig. 4. Differences in the solvation enthalpies between nitro-benzoic and nitro-phenol in various DMSO-water mixtures at 25°C. (A) o-, (0) m-, and (0) p- nitro-phenol.
anions
194
are plotted in Fig. 4. As previously shown [ 11,the p values measure the sensitivity of the phenol derivatives to the electron withdrawing effect of the nitro group with respect to the benzoic derivatives in the same solvent. It is also known [l] that the solvent interferes to a minor extent with the interactions between the reaction site and the substituent group of the nitro-phenols. The pn and ps curves give a more complete picture of the effect of the medium on the reaction constants. At X,,,, = 0.5, where the charge delocalization of the ortho phenol anion is maximum, its solvation is maximum [l]. qkO,PhCOO,,
-(N02PhO_)x,
ACKNOWLEDGEMENT
This work was carried out with the financial support of the Cons&ho Nazionale delle Ricerche (C.N.R.) of Italy.
REFERENCES 1 2 3 4 5 6
F. F. F. F. F. F.
Rodante, Rodante, Rodante, Rodante, Rodante, Rodante,
Thermochim. G. Ceccaroni Thermochim. Thermochim. Thermochim. Thermochim.
Acta, 38 (1980) 311. and M.G. Bonicelli, Thermochim. Acta, 64 (1983) 179. Acta, 23 (1978) 311. Acta, 34 (1979) 29. Acta, 34 (1979) 377. Acta, 32 (1979) 293.